The present invention relates to a rubber composition having excellent heat resistance and oil resistance and good workability, and more particularly to a rubber composition having not only excellent heat resistance, oil resistance and workability but also high modulus which makes it suitable for various types of toothed belts driven under a high load.
Toothed belts are used as power transmission means, and as engine power has been increasing in recent years, the toothed belts have been required to have higher strength to bear an increasingly higher load.
A toothed belt is generally composed of a surface layer made of a woven fabric, etc. and having a toothed shape, a rubber layer for keeping the toothed shape and reinforcing cords, and the toothed belt is subjected to the biggest stress at the bottom of each tooth when meshed with pulley teeth to transmit power. Since this stress is substantially sustained by the rubber layer, the rubber layer should have high modulus so that the toothed belt can withstand a high load.
High modulus can be obtained by using urethane rubbers, but the urethane rubbers are poor in heat resistance and oil resistance. Since many of the toothed belts are used as means for transmitting power generated by engines, heat resistance and oil resistance are extremely important properties. Accordingly, the urethane rubbers are not suitable for toothed belts.
In general, an increase in the amount of carbon black in a rubber composition tends to increase its modulus, but it also increases the Mooney viscosity of the unvulcanized rubber, lowering the workability of the rubber. In addition, the addition of too much carbon black results in higher compression set (smaller rubber elasticity) and larger variation of heat resistance and oil resistance with time.
In view of the above problems, various proposals were made.
Japanese Patent Laid-Open No. 57-12145 discloses a heat-resistant, oil-resistant toothed belt produced by providing an epichlorohydrin rubber between tensile members and the belt back surface, and NBR, CR, SBR or natural rubber in the belt back surface portion and tooth portions.
Japanese Patent Laid-Open No. 57-12146 discloses an oil-resistant toothed belt comprising an NBR portion between tensile members and a back surface, and other belt back surface portions and tooth portions made of CR, SBR or natural rubber.
However, these toothed belts do not have sufficient heat resistance and oil resistance which can meet the levels required recently.
Japanese Patent Laid-Open No. 57-l76343 discloses a toothed belt made of a rubber composition comprising 100 parts by weight of rubber, 0.2-5.0 parts by weight of a silane coupling agent, 10-60 parts by weight of silica and 2-60 parts by weight of carbon black. Further, Japanese Patent Laid-Open No. 57-204352 discloses a power transmission belt comprising a compressed rubber layer made of a rubber-short fiber composite composition comprising 100 parts by weight of rubber, 0.1-5.0 parts by weight of a silane coupling agent, 10-60 parts by weight of a water-containing silicic acid, 2-60 parts by weight of carbon black and 2-40 parts by weight of short fibers. In these rubber compositions, a specific rubber component is a chloroprene rubber. These rubber compositions are excellent in adhesion to tensile cords and wear resistance, but they are poor in heat resistance and oil resistance.
Japanese Patent Laid-Open Nos. 58-78904 and 58-79045 disclose rubber compositions made mainly of EPR or EPDM. These rubber compositions are used for paper-feeding endless belts and conveyor belts, but they do not have sufficient modulus, heat resistance and oil resistance for toothed belts for transmitting engine power.
Further, Japanese Patent Laid-Open No. 58-91947 discloses a high-hardness, high-elasticity rubber composition for toothed belts comprising 100 parts by weight of rubber and 5-50 parts by weight of a high-crystallinity, low-unsaturation thermoplastic polymer. However, this rubber composition has poor properties at an elevated temperature.